As is known, the human knee-joint comprises two pairs of tibio-femoral articular surfaces, namely a medial pair of articular surfaces and a lateral pair of articular surfaces, which are in each case formed by a femoral condyle and by a dish-shaped tibial articular surface interacting with said femoral condyle. It is known (EP-A-1099430) to replace the articular surfaces of the femoral condyles individually by a slide prosthesis and, for this purpose, to use an insertion instrument which guarantees the exact mutual alignment of the prostheses. The prostheses are held on the instrument by means of clamps which are secured on a support plate of the instrument via a releasable coupling. As soon as the prostheses have reached the desired seat in the operation, the coupling is released and the instrument is withdrawn with the support plate from the clamps which remain on the prostheses until the cement used for securing said prostheses has hardened. The releasable coupling consists of a plug on each clamp and of two receiving openings in the support plate, into which openings these plugs can be inserted with an exact fit. They are secured therein by means of screws which remain accessible during the operation so that they can be undone when the instrument is to be removed from the clamps. To be able to insert prostheses at a different predetermined spacing, different support plates are provided in which the receiving openings for the clamp plugs are at a different, predetermined spacing from one another. This has the disadvantage that a large number of different instruments with different support plates have to be kept in stock. Moreover, there are occasions when the operating surgeon wishes to change the spacing of the prostheses, on the insertion instrument being used, during the actual operation. For this reason, another known instrument was created (EP-A-1321116) in which a guide is provided in the support plate for the plugs provided on the clamps, in which guide the plugs and thus the clamps can be moved in the LM direction (LM=lateral-medial) by means of a threaded spindle. This achieved the objective of being able to adjust the spacing during the operation; however, it also had the disadvantage that the clamps are permanently anchored on the support plate by the threaded spindle and that the instrument can be removed from the implanted prostheses only if the clamps themselves are opened, which is not particularly easy.